said by jsz0 :

1) Reasonable base cap based on an average users of a particular system. The ISP should have to publish anonymous data usage stats and clearly define the formula they are using to decide on a cap number.

2) The cap should never effect more than 10%-15% of the users. It should scale based on total average usage.

3) Overage charges should be a flat fee. Perhaps double the monthly price. So if you pay $40 a month for normal capped service, the absolute most you could ever pay is $80. (scaling for different speed packages, of course) I don't think $80 a month is really unreasonable for heavy bandwidth users at all.